The field of the invention is control systems for managing electrical power consumption by industrial and commercial users, and more particularly, load demand controllers for limiting the amount of power consumed during a time interval.
Because power companies charge for "peak" power consumption as well as "total" power consumption, considerable savings can be achieved by limiting the use of electricity during periods of high demand. Control systems for accomplishing this task monitor the power consumption of the user and when a preselected peak, or "demand limit," is approached, one or more non-essential loads are shut down, or "shed." When demand subsides, these loads are brought back on line, or "restored." A basic design objective of such controllers is to minimize the disruption of power to the interruptible loads while at the same time not exceeding the demand limit. In other words, under any given set of circumstances loads are not to be shed unless necessary, the minimum possible number of loads to be shed, and the interrupted loads are to be restored to operation as soon as possible.
There are three basic types of load demand controllers: instantaneous controllers; ideal rate controllers; and predicting controllers. The instantaneous controllers monitor the power usage on a continuous basis and shed loads based on the instantaneous power consumption. Because power companies determine peak demand over a "demand interval" which is typically measured in minutes rather than on the basis of instantaneous demand, the instantaneous demand controller will often shed loads to limit instantaneous demand when it would not have been necessary to do so if the total demand interval had been considered.
The second type of demand controller is the predicting type. Such controllers employ general purpose computers which are programmed to measure the power usage at a predetermined time within each demand interval and based on the preselected demand limit, the time of day, past experience, and other factors, it predicts what the total power demand for that demand interval will be. If needed, loads will be shed or restored to maintain power consumption near the demand limit. In addition to the high cost of the hardware for such predicting type demand controllers, the programming of them is time consuming and costly.
The third general type of demand controller is the ideal rate type controller. In such a controller pulses are received from the electric meter at a rate proportional to power consumption. This pulse rate is compared with an "ideal" pulse rate, which if maintained throughout the demand interval would exactly equal the demand limit. The loads are shed or restored based on the difference between the ideal rate and the actual power consumption rate. For each load connection to the controller a shed rate and a restore rate is established and an offset adjustment is provided to inhibit load shedding early in the demand interval. The "programming" of such a demand controller requires considerable information about each load device and a number of calculations using this information must be made. The meaning of the various settings and readouts on the controller is far from obvious and requires considerable study.